Although old adults are often more fatigable than young adults, this is not a universal finding. As suggested in an exploratory workshop by the National Institute of Aging (NIA) on unexplained fatigue in the elderly, such differences in performance between young and old adults may be related to muscle energetics. We propose to compare the distribution of activation and energy utilization among a group of synergist, antagonist, and accessory muscles in young and old men during two types of fatiguing contractions with the elbow flexor muscles. The difference between the two contractions will be the type of load supported by the arm muscles: one fatiguing contraction will require an individual to sustain a submaximal force (force task) and the other fatiguing contraction will involve supporting an equivalent inertial load and keeping the position of the arm constant (position task). Although the net muscle torque exerted during the two contractions will be similar for each subject, the position task is more difficult and cannot be sustained for as long as the force task. The central hypothesis of the research plan is that differences in energetics of arm and accessory muscles during the two tasks can explain the age-related differences in times to task failure. Positron emission tomography (PET) will be combined with computer tomography (CT) to measure blood flow and oxygen consumption in arm muscles during force and position tasks, and PET images will provide information about the glucose uptake in individual muscles to estimate cumulative levels of muscle activity. The research plan involves first comparing blood flow and oxygen consumption in arm muscles of old and young men during two types of fatiguing contractions (Aim 1) and then comparing glucose uptake in arm and accessory muscles of old and young men after two types of fatiguing contractions (Aim 2). Although the relative performance of young and old men during a position task is unknown, we expect to explain any differences in time to failure for both tasks by differences in muscle energetics. By understanding the task dependent role of muscle energetics, specific interventions can be developed to maximize fatigue resistance in older adults. Public Health Relevance: Young and old men will perform two types of fatiguing contractions that differ in difficulty. The plan is to determine if differences in performance between the two groups of participants can be explained by variation in blood flow and energy consumption in the involved muscles.